X-ray structrual investigations are being carried out with the aim of studying a number of important problems. Work is being carried out on a number of molecular analogs of dincleotides, where the sugar and phosphate linkages have been repalced by a three-carbon bridge, or by a hydroxy-tetrahydrofuran ring. These studies should cast light on the preferred conformations of dinucleotides and have important consequences on our knowledge of the detailed structure of nucleic acids. Structual studies are also being undertaken on some epsilon- modified nucleoside salts. X-ray analyses are also proposed on several key molecules - vernamycin (metal-free), and derivatives of the poly-oxygen antibiotic, X-537A -- which are involved in metal ion transportproblems. A major object of this work is to discover the conformational changes that take place upon uptake of metal ions. This aspect of the research will involve the application of the symbolic additionmethod to molecules with more than 60 non-hydrogen atoms, and which crystallize in non-centrosymmetric space groups. The detailed structure of several polyene and oligopeptide antibiotics will also be probed by x-ray diffraction methods. In general, very little is known about these structures and combined X-ray diffraction and mass- spectrometric investigations offer the most efficient means of providing such information.